Polymer latex has been used extensively for silver halide photographic products. For example, polymer latexes have been employed in several applications in which physical property modification was advantageous or essential. Properties that can be affected include dimensional stability, flexibility, drying rate, cracking, abrasion resistance, and differential swelling. The maintenance of dimensional stability is especially important in graphic arts products, microfilm, X-ray products, and other applications with a premium on resolution. Incorporation of a hydrophobic, nonswelling phase proportionately reduces swelling as a function or relative humidity and thus the drying will be also improved. Low glass transition (Tg) latexes and gelatin grafted latex have been used for the reduction of pressure sensitivity or undesirable fog of the photographic products. Polymer latexes are especially useful as the carrier of the photographically useful compounds. The photographically useful compounds include image couplers, developers, masking couplers, antifoggants, brightners, lubricants, latent image stabilizers, sensitizers, filter dyes, UV absorbers, oxidized developer scavengers, hardeners, stabilizers, antioxidants, bleach accelerators, and coupler solvents. This process is often called "loading". There are two major processes for the loading of photographically useful compounds into polymer latexes. In the first process, the photographically useful compounds, coupler solvents, such as o-dibutylphthalate, and a low boiling water-miscible organic solvents, such as tetrahydrofuran, acetone, or methanol are mixed to form a homogeneous solution. This solution was then added slowly to the polymer latex with vigorous stiring to force the loading of photographically compounds into polymer latex. The low boiling organic solvents is removed by evaporation after the loading is completed, with the result that the hydrophobic compounds becomes imbedded in the latex particles. The first process is described in U.S. Pat. Nos. 4,203,716, 4,304,769, and U.S. Pat. No. 4,368,258. In the second loading process the photographically useful compounds, such as couplers, and optional high-boiling solvents, such as o-dibutylphthalate, are combined at a temperature sufficient to prepare a liquid solution of the oil compounds. This oil solution is then combined with an aqueous solution containing gelatin and surfactants to form pre-mix. Polymer latex is either included in the aqueous solution before the oil phase is added, or is added to the oil-gelatin premix. The mixture is then passed through a high shear device, such as homogenizer, colloid mill, or microfluidizer, to force the loading of the photographically useful compounds into the polymer latex. The second loading process is described in U.S. Pat. No. 5,594,047, EP 0 727 703 and EP 0 727 703. The second process is preferred since low boiling organic solvents are not required.
The particle size of the polymer latexes used in the photographic products are usually under 100 nm or preferably under 50 nm. Larger particle size polymer latexes tend to scatter light and reduce the film clarity of the photographic products. Large particle size latex also reduce the loading efficiency and reduce the photographic performance of the photographically useful compounds. However, gelatin solution containing small particle size latexes or dispersions tend to have very high melt viscosities. The cause of the high melt viscosity of small particle size latexes or dispersion is due to adsorption of gelatin onto the surface of these particles and thus the apparent hydrodynamic volume of the particles increase dramatically. The schematic explanation of this phenomena was shown column 4 lines 3-4 of U.S. Pat. No. 5,135,844. High melt viscosity not only cause manufacturing difficulties and reduced the coating speed but also caused coating defects, such as coating unevenness and coating streaking. This is especially true when the particle size of the hydrophobic latex is less than 50 nm.
There are few addenda known in the prior art for the reduction of high melt viscosity of small particle size latex or dispersions. U.S. Pat. Nos. 3,409,435, U.S. Pat. No. 5,135,844, and U.S. Pat. No. 5,300,418 describe the use of oligomeric surfactants for the reduction of high melt viscosity. Three types of surfactants which are effective to reduce the high melt viscosity are mentioned. The first type is a surfactant composed of a 6 to 22 carbon atom hydrophobic tail with one or more attached hydrophilic chains of at least 8 oxyethylene and/or glycidyl ether groups that may or may not be terminated with a negative charge such as a sulfate group. The second type are block oligomeric surfactants composed of hydrophobic polyoxypropylene blocks(A) and hydrophilic polyoxyethylene blocks(B) joined in the manner of A-B-A, B-A-B, A-B, or with a connecting moiety between them. The third addenda type are sugar surfactants, composed of between one to three 6 to 22 carbon atom hydrophilic tails with one or more attached hydrophilic mono or oligosaccharidic chains that may or may not be terminated by a negatively charged group such as a sulfate group. However, such prior art materials that containing a large number of polyalkylene oxide groups produce adverse photographic (sensitometric) effects in some photographic products. These oligomeric surfactants tend to migrate from layer to layer or be adsorbed to the silver halide grain which fogs the emulsion. Sugar surfactants do not have the adverse photographic effects but they tend to reduce the surface tension of the layer and create coating problems, such as repellency. EP 0 695 968 discloses the use of a-cyclodextran for the reduction of melt viscosity. However, .alpha.-cyclodextran is expensive and fairly large amounts are needed to be effective. Another way of reducing high melt viscosities is by dilution with water. However, such a procedure leads to increased water load in the drier, and reduced the drying time.
Therefore, there is a need for the alternative ways to reduce the high melt viscosity when small particle size polymer latexes are used in photographic systems.